1. Field of the Invention
The present invention relates to a suck back valve which, for example, prevents liquid drip from occurring in a fluid supply port, by sucking back a fluid which flows through a fluid passage in accordance with a displacement action of a diaphragm, yet which is capable of stabilizing the suck back amount of the fluid.
2. Description of the Related Art
The suck back valve has been hitherto used, for example, in a production process for forming semiconductor wafers. The suck back valve has a function to prevent so-called liquid drip, i.e. a phenomenon in which a minute amount of coating liquid drips toward the semiconductor wafer from a supply port when the supply of coating liquid to the semiconductor wafer is stopped.
A suck back valve according to a conventional technique is shown by FIG. 6, further details of which may be seen, for example, in Japanese Utility Model Publication No. 8-10399.
The suck back valve 10 includes a main valve body 18 formed with a fluid passage 16 for making communication between a fluid-introducing port 12 and a fluid-discharging port 14, and a bonnet 20 coupled to an upper portion of the main valve body 18. A diaphragm 26, which comprises a thick-walled section 22 and a thin-walled section 24, is provided at a central portion of the fluid passage 16, wherein the overall surface of the thick-walled section 22 and thin-walled section 24 of the diaphragm is formed so as to face the fluid passage 16.
A piston 30 is fitted to the diaphragm 26, wherein a v-packing 32, which is slidable on an inner wall surface of the main valve body 18 and which functions as a seal, is installed on the piston 30. Further, a spring 34, which constantly presses the piston upwardly, is provided in the main valve body 18. A pressurized air supply port 28 is formed in the bonnet 20. Reference numeral 36 illustrates an adjustment screw abutting with the piston 30, for adjusting a flow amount of coating liquid to be sucked back by the diaphragm 26, based on adjustment of a displacement amount of the piston 30.
Further, reference numeral 33 shows a flow amount control valve, wherein a pressurized air supply source 35 is connected to the pressurized air supply port 28 through the flow amount control valve 33. In addition, a coating liquid supply source 38 is connected to the fluid inlet port 12 through the ON/OFF valve 37.
An explanation of operation of the suck back valve 10 constructed as described above, shall now be given in outline. First, the ON/OFF valve 37 assumes an ON state, and coating liquid is supplied toward the fluid outlet port 14 from the coating liquid supply source 38 and via the ON/OFF valve 37 and the fluid inlet port 12. At this time, the flow amount control valve 33 is controlled, and as pressurized air is supplied from the pressurized air supply source 35 to the pressurized air supply port 28, the piston 30 is displaced downward by the pressure of the pressurized air, wherein the diaphragm 26 connected to the piston 30 projects into the fluid passage 16, as shown by the two-dot-chain line in FIG. 6.
When the ON/OFF valve 37 assumes an OFF state, the flow of coating liquid in the fluid passage 16 is halted, whereas when the flow amount control valve 33 is controlled and supply of pressurized air to the pressurized air supply port 28 from the pressurized air supply source 35 is halted, a condition is reached in which the piston 30 and diaphragm 26 are raised in unison under an action of the elastic force of the spring 34. An end of the adjusting screw 36 abuts with the piston 30, and along with controlling the displacement thereof, the diaphragm 26 is restored to the position shown by the actual lines in FIG. 6, resulting in lowering of the pressure in the passage 16. More specifically, a predetermined amount of coating liquid remaining in the fluid passage 16 is sucked by a negative pressure action of the diaphragm 26, whereby liquid dripping at the coating liquid supply port connected to the fluid outlet port 14 side is prevented.
Notwithstanding, in the above-described conventional suck back valve 10, a flow amount control valve 33 for controlling an amount of pressurized fluid supplied to the pressurized air supply port 28 becomes necessary, and moreover, pipe connecting operations for interconnecting the suck back valve 10 and the flow amount control valve 33 add complexity. Further, there is a disadvantage that a dedicated space for the system employing the suck back valve becomes large. Further, because pressurized air supplied to the pressurized air supply port 28 is controlled by the flow amount control valve 33, whereby the piston 30 is moved and the diaphragm 26 is displaced, time is required from initiating control of the flow amount control valve 33 until actual displacement of the diaphragm 26 occurs, resulting in a delay in the response speed of the diaphragm 26. Accordingly, the delay in operation from halting of coating liquid supply until the coating liquid is sucked back causes trouble, so that more than a predetermined amount of coating liquid drips onto the semiconductor wafer, with the concern that the expected goal of preventing liquid dripping cannot be achieved.
Furthermore, because the diaphragm 26 is displaced by pressurized air supplied to the pressurized air supply port 28, pressure of the pressurized air supplied from the pressurized air supply source 35 tends to vary, causing a change in the displacement amount of the piston 30, and in turn the amount of coating liquid sucked by the diaphragm 26 also changes. As a result, when supply of coating liquid is halted, coating liquid remaining inside the fluid passage 16 is not sufficiently sucked, with the fear that unwanted dripping of coating liquid onto the semiconductor wafer may occur. On the other hand, if more than the predetermined amount of coating liquid is sucked back, when the suck back valve 10 is used again and supply of coating liquid is reinitiated, there is the problem that additional time is spent until coating liquid begins dripping onto the semiconductor wafer.
Still further, the suck back valve 10 and the ON/OFF valve 37 have to be disposed separately from one another, and a pipe connecting operation for interconnecting the suck back valve 10 and the ON/OFF valve 37 becomes necessary. Hence, there is the defect that a dedicated space for installation of the system employing the suck back valve 10 becomes increasingly large.